The common consent argument for the existence of God states that if most people believe God exists, then he does exist. It is a form of argumentum ad populum (appeal to majority).

"As the human intellect, though weak, is not essentially perverted, there is a certain presumption of the truth of any opinion held by many human minds, requiring to be rebutted by assigning some other real or possible cause for its prevalence. And this consideration has a special relevance to the inquiry concerning the foundations of theism, inasmuch as no argument for the truth of theism is more commonly invoked or more confidently relied on, than the general assent of mankind."

Variant: fastest growing religion

"Islam is the largest spreading religion in the western world and not because Muslims have more children than others. More than 20000 Americans convert to Islam each year. 4000 have converted to Islam in Germany this year alone. Thousands are converting to Islam in Latin America. WHY? [3]"

Which God?

There are large numbers of believers in different religions. Since multiple incompatible conclusions are supported by the argument it is a broken compass argument.

Naturalistic explanation

Humans may have some commonly occurring beliefs but that does not make that rational or true. The fact that religion occurs throughout history and in all cultures might be due to natural processes, such as human psychology and cognition produce tendencies to hold certain beliefs.

Recognised Authority Exception

One special case is that in which a statement is said to be true because it is believed by most of the experts in the field (9 out of 10 dentists recommend Brand X toothpaste!). For example, if most astronomers say that the Earth revolves around the Sun instead of the other way around, then that is very likely to be true. In this case, however, we are trusting the judgment of people who have carefully studied the matter. In effect, we are trusting that the experts have reached their conclusions through valid arguments based on careful observation, so there is no need for us to research the matter ourselves. This type of argument is often reliable, but not always. After all, scientific knowledge is never perfect and complete. However, for most "mature" scientific fields, the likelihood of a complete reversal of views — such as moving the Earth from the center of the universe to the outskirts of one unremarkable galaxy among millions — is incredibly, and ever increasingly, small.

Within the ad populum fallacy is the fallacy of appealing to authority, since this argument assumes that the majority hold the authority regarding truth. Counter to the example of the 10 dentists, theological belief is not a skill developed by professionals of the field; rather, it is something accessible to all upon reflection. Under this framework, appealing to theological authority (i.e. the authority of those claiming Christianity to be true) is another fallacy within this argument.

Arrogance of disagreeing with majority view

Some apologists argue it is arrogant to dissent from the majority view regarding the existence of God. However, their religion was not always in a majority. Does that make it untrue during this time?

"Isn't it somewhat arrogant to suggest that countless churches and people (including men like Abraham Lincoln) are all radically in error in their view of the Bible? [5]"

Also, the accusation of arrogance is an ad hominem argument that has nothing to do with the existence of God.

Variant: argument from the consensus of mystics

Many variants of the argument could be proposed that consider a limited group of people. Mystics might be more "qualified" to comment on religion but this is an argument from authority. [4]

However, this variant suffers from the same problems as the original argument. It is rather like saying "astrology is true because most astrologers believe it".

Also, the experiences of mystics probably can be explained by natural processes.

Variant: most scientists believe in God

In a survey of US academics, Elaine Howard Ecklund reported that scientists hold the following beliefs: 34% atheists, 30% agnostics, 8% in a higher power, 25% were conventional theists. [7] A survey of the members of the American Association for the Advancement of Science found 33% theists, 18% believe in a higher power, 41% don't believe in either, 7% don't know/refused.[8]

The RASIC international study of scientists' beliefs found that beliefs varied by country: for example India, 27% believe in God, 38% in a higher power, 6% non-religions. In contrast, UK reported 65% non-religious scientists. On the whole, scientists are less religious than the general population but there are regional exceptions (such as Hong Kong and Taiwan). [9][10]

In light of the survey evidence, it is hard to argue that the majority of scientists hold religious beliefs, but even if they did, it would not prove much. This argument is related to the argument from admired religious scientists.

Variant: most intelligent people believe in God

"Let’s face it: most intelligent people believe in God, as did most world leaders in the past.[11]"

Even intelligent people, when raised from birth and surrounded by fellow believers, have a hard time shaking off irrational beliefs.[12]

Religions are popular because the truth would be apparent

"omit those religions [...] that are not widespread, because usually the truth of a right religion is so apparent that many would follow it. [13]"

That truth would be apparent and that people would seek the truth are unsupported claims.